NEW YORK — To most people in the Northern Hemisphere, May Day conjures images of brightly colored twirling ribbons and promises of warm days ahead. That’s not the whole story, though: May Day is also a day of protests and riots that traces its modern roots back to a world-changing explosion in Chicago.

When is May Day?

May Day is May 1 every year. Easy to remember, right?

What is May Day?

Depending on where you are, it’s either a seasonal celebration or a day to celebrate workers’ rights, or maybe a little bit of both. Think of the latter use as a Labor Day, if you will, for the rest of the world.

How did it start?

This is a more complicated question. Originally, May Day was an ancient pagan holiday celebrating the start of summer. In Gaelic traditions, it is known as Beltaine (or the Anglicized “Beltane”). As time went on, different groups adapted the celebration to their specific cultures or beliefs. Europeans and Americans often celebrate in a more secular manner with diversions like maypole dancing and flower crowns. (That certainly lends a bit of cultural context to all the young women breezing around summer music festivals this time of year with giant daisies on their heads.)

Also of note: In May, the Southern Hemisphere is getting ready for winter, so May Day as a seasonal celebration is, for the most part, a Northern Hemisphere thing.

How did it become a day for labor rights?

May Day is also a labor holiday in many areas of the world, and that part of its history is a thornier story. May Day has shared a date with International Workers’ Day since the 1880s. At the time, labor movements around the world were fighting for fair work accommodations like eight-hour workdays and unions. The date was chosen because it aligned with the anniversary of the Haymarket affair in Chicago, where police killed four people at a peaceful protest after someone threw a bomb into the crowd.

The event had a huge impact on labor movements across the world.

So why are there sometimes riots and marches on May Day?

Because of its more recent history, International Workers’ Day/May Day is often a day of protest for labor unions around the world. The people come out to rally, and sometimes their passionate demonstrations can turn violent. In 2014, Turkey attempted to ban labor rallies, citing security concerns. Across Europe, similar events have attracted heavy police presence.